A flowmeter is used to measure the flow rate of fluid such as kerosene, water, gas or the like consumed at home or business enterprise. A thermal type (particularly, indirectly heated type) flow sensor which is easily reduced in cost has been used in the flowmeter.
There is known such an indirectly heated type flow sensor that a sensor chip comprising a thin film heating element and a thin film temperature-sensing element which are laminated through an insulating layer on a substrate by using a thin film technique is disposed so that heat transfer (that is, thermal interaction) is allowed between the sensor chip and fluid in a pipe (containing a fluid flow passage provided in a flowmeter so as to intercommunicate with an outer pipe). The temperature-sensing element is heated by making current flow into the heating element to vary the electrical characteristic, for example, the value of electrical resistance of the temperature-sensing element. The variation of the electrical resistance value (based on increase in temperature of the temperature-sensing element) is varied in accordance with the flow amount (flow rate) or flow velocity of fluid flowing in the pipe. This is because a part of the heating value of the heating element is transferred to the fluid, the heating value diffused into and absorbed by the fluid is varied in accordance with the flow amount (flow rate) or flow velocity to thereby vary the heating value to be supplied to the temperature-sensing element. Therefore, the electrical resistance value of the temperature-sensing element is varied. The variation of the electrical resistance value of the temperature-sensing element is made different in accordance with the temperature of the fluid. Therefore, there has been utilized such a manner that a temperature sensing element for temperature compensation is installed in an electrical circuit for measuring the variation of the electrical resistance value of the temperature-sensing element so that the variation of the measured flow-rate value due to the temperature of the fluid is suppressed to the minimum level.
Such an indirectly heated type flow sensor using a thin film element as described above is disclosed in JP-A-11-118566, for example. This flow sensor uses an electrical circuit containing a bridge circuit for achieving an electrical output corresponding to the flow rate of fluid.
In the flowmeter as described above, a fin plate for the heat exchange between the sensor chip and the fluid is projected into the fluid flow passage, and an electrical circuit portion containing a circuit board for calculating the flow rate, a display portion, a communication circuit connecting portion and other parts are disposed at the peripheral portion of the flow sensor, and all the function portions of the flowmeter containing the above parts are accommodated in the housing.
In a flowmeter using an indirectly heated type flow sensor, a part of the heating value occurring in the heating element (heater) is transmitted to the fluid, and on the basis of the fact that heat absorption is performed in accordance with the flow rate or flow velocity of the fluid, the output value of the electrical circuit which corresponds to the heat absorption amount is converted to the flow rate value by using a calibration curve. The calibration curve is achieved on the basis of an experiment or the like which was beforehand performed on fluid to be subjected to the flow-rate measuring. Accordingly, when the fluid to be subjected to the flow rate measuring and environmental conditions in the flow rate measuring are the same as those used when the calibration curve was created, substantially no error occurs in the flow-rate measuring when the conversion using the calibration curve is carried out.
However, the fluid formed of mixture of plural kinds of molecules having different molecular weights such as kerosene or the like is varied in volume in accordance with the temperature. Therefore, an error may be contained in a flow rate value achieved by the conversion using a calibration curve when the detection was carried out at a temperature different from that when the calibration curve concerned was created.
Therefore, the present invention has an object to perform flow-rate measuring having little measurement error even when the volume of fluid is varied due to variation of the temperature. Particularly, the present invention has an object to provide a method and a flowmeter for the flow-rate measuring as described above.